LG V20 signal issues. How To Fix?

Just purchased a brand new AT&T unlocked LG V20 phone. It was network unlocked by the previous owner and that's the only time they used the phone before selling it. I am an AT&T user so I figured the phone would still work fine for me.

Upon getting the phone, signal in my house has become extremely spotty. With my old Galaxy S6 I always had at least two bars of signal and always had 4G LTE.

Sometimes it will be connected to the AT&T network and sometimes you won't see the AT&T letters listed at the top left of the phone, but youll have 2 bars of signal. When this happens and I try to check my e-mail or use data, I get a message saying Network Unavailable. The phone often seems to change towers and sometimes will display the AT&T letters at the top left of the screen and sometimes won't. When it doesn't it says Network Unknown. I am using my old sim card from my Galaxy S6. It's a two year old sim card. I contacted AT&T about the issue and they are sending me a brand new SIM card for the phone. I just have to activate it upon arrival by calling them.
I've checked APN settings and they are still set properly for the AT&T network. I've tried a factory reset, turning airplane mode on and off, and even signal booster apps. Nothing has worked.
Ive been trying methods here on this article: